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- Marble (2)
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The present study documents the results of an
inter-disciplinary model project that was planned with the
aim of developing an innovative winter covering system
for marble statuaries located on the Schlossbru¨cke (Berlin).
Such a system would need to fulfil the various requirements
for structural stability, aesthetics, climate and practical
use. This applied research represents the first complex
scientific study of the sustainability of a winter covering
system. The study is characterised by the use of complex
scientific instruments such as special laboratory analysis
and numerical simulation tools. The interaction between
the environment and the artefacts in connection with the
innovative winter covering structures were studied by
extensive climatic monitoring.
Für die bisher der winterlichen Witterung schutzlos ausgelieferten acht Figurengruppen auf der Schlossbrücke in Berlin wurde innerhalb des vom Landesdenkmalamt Berlin und der Senatsverwaltung für Stadtentwicklung initiierten Forschungsprojektes geprüft, ob und welcher Typ einer Einhausung in der Lage ist, die schädlichen Umwelteinflüsse zu reduzieren. Um den hohen konservatorischen, bauphysikalischen, bautechnischen, künstlerischen und stadträumlichen Anforderungen gerecht zu werden und mögliche Lösungen erstmalig umfassend auf ihre Effektivität zu untersuchen, war die Zusammenarbeit von Denkmalpflegern, Restauratoren, Architekten, Ingenieuren, Geologen, Materialwissenschaftlern und Bauphysikern notwendig. Das durch die Deutsche Bundesstiftung Umwelt (DBU) geförderte Projekt zielte darauf ab, in einem Modellvorhaben mit seinen Partnern, der Technischen Universität Dresden (Institut für Bauklimatik), der Universität Göttingen (Abt. Strukturgeologie und Geodynamik), der Bundesanstalt für Materialforschung und -prüfung (Fachgruppe 7.1, Baustoffe) und unter Beteiligung der Universität der Künste (Fachbereich Konstruktives Entwerfen und Tragwerkslehre) in Berlin, eine innovative und nachhaltige Wintereinhausungskonstruktion auf die besondere Situation der Marmorskulpturen der Schlossbrücke anzupassen und ihre Wirksamkeit zu überprüfen.
Since ancient times, marble has been the preferred material for monuments, sculptures, Ornaments and architecture. Though the stone is often a Chosen material, long-term exposure of marble results in cumulative deterioration of the rock fabric. The rate and extent of deterioration depends on the rock fabric and the climatic conditions. Besides the thermal vulnerability of marble, a combination of thermal and hygric fluctuation accelerates the deterioration process. The weathering sensitivity of marbles can be characterised by the irreversible length change of samples after heating under thermohygric conditions as residual strain. This residual strain is a non-reversible deterioration and caused by microcracking induced by a pronounced anisotropy of the thermal dilatation coefficient of calcite.
In brittle materials like marble, cracking and crack growth or friction on crack surfaces are accompanied by release of acoustic waves. The analysis of these acoustic emissions can give a deeper insight into the deterioration mechanism of marble.
In this study, acoustic emissions of thermohygric treated marble were analysed and correlated with ultrasonic velocities, thermal dilatation and residual strains. Therefore, different types of calcitic marble were cyclically heated from 20 °C to 90 °C and after equilibration of the samples cooled down again to 20 °C. While the first cycles were performed under dry conditions, the following were
executed in a humid environment. The analysis of acoustic emissions enables one to determine when cracking occurs during the thermal treatment. It is also possible to differentiate microcracking from internal friction. Furthermore, the evolution of Deterioration can be estimated based on ultrasonic velocities. The combination of acoustic Methods and strain measurement gives an insight into the disintegration mechanism and Supports the development of prevention strategies.
The long-term exposure of marble objects leads to a successive deterioration of the rock fabric and may cause severe damage on cultural heritage objects. The magnitude of disintegration depends on the rock fabric as well as on the climatic conditions. Some types of marble show no considerable damage even if they have been exposed in northern Europe for several decades or centuries. In contrast, other marble varieties are showing severe damage after a few years of weathering. The present study shows that marble decay, along with the initial thermal degradation, is mainly influenced by the hygric and thermo-hygric load, respectively.
The influence of the rock fabric on the resistivity against weathering is also documented. The factors causing marble decay were compared to extensive climate data, which were collected during a scientific project that was started with the aim to develop a covering system for marble sculptures in Berlin. Based on the hygrothermal material properties of the marble and on the climatic conditions, the heat and moisture Transport inside the material and hence the hygrothermal load changes were calculated with a high spatial resolution. The calculations showed that the temperature and moisture fluctuations inside the material are mainly controlled by the dimensions and orientation of the sculptural elements.
The current state of preservation was investigated by two-dimensional ultrasonic tomography. The results show a good correlation to the calculated distribution of the hygrothermal load. Based on this finding, requirements on a protective winter shelter system for the sculptures were defined.
The project results show that in addition to protective covers, regular inspection and maintenance combined with regular cleaning ensures an effective and sustainable protection of marble objects in northern Europe.